APPARATUS FOR COUPLING ELECTROMAGNETIC RADIATION TO AN ELECTRODELESS LAMP

Abstract: This invention pertains to an apparatus for coupling electromagnetic energy to electrodeless lamps. A waveguide (20) having one end connected to a source of electromagnetic radiation (23) was closed at the second opposite end (18). A coupling device at the second end (18) couples microwave energy from the waveguide (20) to an electrodeless lamp (11). The coupling device includes a coaxial transmission line (12) having a center conductor (15) extending through one of the walls of the waveguide (20) adjacent the second end (18). An alcove partition (27) within the waveguide (20) contacts the extending conductor (15) and forms an alcove (19) in the waveguide (20). The alcove portion (19) provides for an impedance matching structure between the transmission line (12) and waveguide (20). The electrodeless lamp (11) which is positioned above the free end of the coaxial transmission line (12) is excited with the coupled microwave energy.

Description: APPARATUS FOR COUPLING ELECTROMAGNETIC RADIATION TO AN ELECTRODELESS LAMP Technical Field The present invention relates to microwave-excited electrodeless lamps. Specifically, an apparatus for coupling microwave energy to the electrodeless lamp is described. Background of the Invention Electrodeless lamps are used in various applications wherein the longevity of the lamp is a paramount consideration. Such lamps include a sealed translucent envelope containing a gas which can be excited by electromagnetic radiation to generate high intensity white light. The devices receive electromagnetic energy from a microwave signal which is coupled from a standard magnetron microwave source. A recent application for high-intensity lamps is in the field of projection television systems. These systems require a source of high intensity white light. The white light is separated into the primary red, green and blue colors, each of which is modulated with appropriate red, green and blue (R G B) signals. The modulated red, green and blue images are combined in conventional dichroic mirror structures to produce a composite color image. A projection lens generates an enlarged display image from the magnified composite signal. Such devices operate for extended periods of time. Conventional projection television systems rely upon either high intensity discharge arc lamps, or CRT devices which are operated at high electron potentials. These devices have a limited operational life, and a consumer may well need to replace these high-intensity light generators several times during the lifetime of the television system. The electrodeless lamp technology offers the promise of implementing high-intensity light sources with a life expectancy far exceeding the life expectancies of these other prior art light sources. Sufficient light intensity can be generated from a single electrodeless lamp which is used in conjunction with a conventional reflector structure to distribute the light over the aperture of an optical system for producing the red, green and blue images. The optical requirements for projection dictate that the light source must be small, on the order of 5 mm. diameter. A disadvantage of using the electrodeless lamp in this application includes the requirement that they be microwave-excited. The microwave source must generate microwaves having a power level of 100-400 watts, depending on the projector. Sufficient microwave energy must be coupled to the electrodeless lamp where it is converted into radiant white light. The small size of the lamp requires intense electric fields to couple the energy to the lamp. These power levels produce high levels of heat, requiring that the lamp be cooled by a stream of gas, such as compressed air. The complications associated with exciting an electrodeless lamp with microwave energy include the requirement that a broad-band low reflection coupling be provided between the microwave source and the lamp. Otherwise, the operating frequency tolerances which accompany different microwave sources, such as magnetrons, may produce an unmatched condition which produces microwave reflections which are received in the magnetron. These reflections shift the frequency of the magnetron, producing further losses in efficiency and a corresponding loss of light output. The present invention is directed to an apparatus which will couple microwave energy from a standard microwave source to an electrodeless lamp with a small reflection coefficient over a bandwidth representing the frequency tolerance of commercial magnetrons. Summary of the Invention It is an object of this invention to provide a coupling device for coupling microwave energy to an electrodeless lamp. It is a further object of this invention to provide for a projection television light source using an electrodeless lamp. It is yet a more specific object of this invention to provide for a cooled electrodeless lamp structure which is excited from a microwave source. These and other objects of the invention are provided by a waveguide structure which supports a TE 10 mode microwave signal, coupled at one end to a source of microwave energy, and closed at a second end. A coupling device is provided at the closed end for coupling microwave energy within the waveguide to an electrodeless lamp. The coupling device includes an opening in a wall of the waveguide structure adjacent the closed end, which faces an alcove formed by a partition which occludes a major portion of the waveguide cross-sectional area, leaving a minor amount of area which defines the alcove. The alcove extends from underneath the opening in the wall of the waveguide to the closed end of the waveguide. One end of the center conductor of a coaxial transmission line structure is inserted through the opening into the alcove, and makes electrical contact with the alcove partition. The other end of the center conductor is positioned underneath the electrodeless lamp. A substantially transparent coaxial outer conductor is connected to the exterior of the waveguide wall. The device provides for a substantially broadband coupling loop between the coaxial transmission line and the rectangular waveguide structure over a 10% bandwidth, impedance matching the coaxial transmission line structure terminated with the electrodeless lamp to the waveguide structure. Description of the Figures Figure 1 illustrates a light source for a projection television which uses an electrodeless lamp. Figure 2 is a section view of a preferred embodiment of the invention for coupling electromagnetic energy from a waveguide structure to an electrodeless lamp. Figure 3 is a top view of the coupling device of Figure 2. Figure 4 is a section view of yet another embodiment of a coupling device in accordance with the invention. Figure 5 is a top view of the device of Figure 4. Figure 6A is a section view of the center conductor of the coaxial transmission line of the embodiment of Figure 1. Figure 6B is a top view of the center conductor of the coaxial transmission line of Figure 1. Figure 7A is a section view of yet another embodiment of the device in accordance with the invention. Figure 7B is a top view of the additional embodiment of Figure 7A. Figure 8 is a section view of another embodiment of the invention. Figure 9 illustrates the electrical coupling between the free end of the center conductor of the coaxial transmission line and the electrodeless bulb. Figure 10 is a schematic drawing illustrating the electrical coupling between the coaxial transmission line and the waveguide. Figure 11 is a schematic drawing of the transmission line including an electrodeless lamp which produces a reflection coefficient which is to be matched by the waveguide coupling device. Description of the Preferred Embodiment Referring now to Figure 1, there is shown a light source for generating a high intensity white light, especially for use in projection television applications. The device includes an electrodeless lamp 11 as a light-emitting element. The electrodeless lamp 11 is supported on a rotating shaft 17, driven by motor 16. The lamp 11 is rotated greater than 8,000 RPM to facilitate cooling of the lamp structure, as well as to uniformly excite the gas within the electrodeless lamp. The electrodeless lamp 11 is excited by microwave electromagnetic energy which exits a coaxial transmission line structure 12 having a center conductor 15 and an outer conductor 14. The coaxial transmission line structure is coupled to a waveguide 20. The waveguide 20 is in turn connected through an isolator 22 to a magnetron 23. Light from the electrodeless lamp 11 passes through the transparent outer conductor structure 14 which may be a cylindrically-formed wire mesh, and is incident to a reflector 13. The reflector 13 has an aperture co¬ extensive with the entrance aperture of the optical system of a projection television. The magnetron 23 has a frequency in the ISM microwave band which is centered at 2450 MHz. An.isolator 22 effectively isolates any energy reflected from the waveguide section 20 which may shift the frequency of operation of magnetron 23 away from a nominal frequency. As the waveguide 20 is matched in a particular frequency range to deliver maximum microwave power to the lamp 11, any frequency tolerance associated with the magnetron 23 could result in a sufficient reflection being returned from waveguide 20 such that the frequency of the magnetron 23 is pulled from its nominal frequency further increasing the size of the reflection. Increases in reflected energy consequently reduce the amount of energy delivered to a load. The coupling of electromagnetic energy from the waveguide 20 to the electrodeless lamp 11 is provided by a transmission line structure comprising a center conductor 15 and outer conductor 14. The center conductor 15 passes through an opening in the waveguide 20 into a coupling chamber 19 defined as an alcove formed at the end of the waveguide 20. The section of center conductor 14 which is exposed in the alcove 19 forms a coupling loop. The alcove 19 is shaped to provide for an impedance match between the coaxial transmission line defined by center conductor 15 and outer conductor 14 to the waveguide 20. The waveguide 20 is terminated at the second end by a short 18. The center conductor 15 is hollow and exits the waveguide through a clearance hole, spaced from the upper wall of the waveguide 20 to avoid arcing therewith. The other end of the center conductor 15 extends through the partition 26, defining the alcove, and exits through the opposite side of the waveguide 20. The hollow center conductor 15 is connected to a source of compressed air 25 and supplies cooling air to the surface of the electrodeless lamp 11. The microwave circuit, comprising the waveguide 20, alcove 19 and coaxial transmission line 12 couples the magnetron-produced microwave energy to the electrodeless lamp 11, causing it to emit high-intensity white light. A motor 16 connected to shaft 17 rotates the electrodeless lamp 11 so that cooling air uniformly cools the surface of the electrodeless lamp 11, and uniformly illuminates the electrodeless lamp 11 with microwave energy. The outer conductor 14 of the coaxial transmission line 12 is transparent to light and, in a preferred embodiment, comprises a mesh conductor, terminating on the upper wall of waveguide 20, extending above the electrodeless lamp 11. The outer conductor 14 mesh extends above the electrodeless lamp 11 to shield significant levels of radio frequency energy from being radiated by the transmission line. Figure 2 illustrates in greater detail the structure of the coupling device connecting microwave waveguide 20 and transmission line 12. The alcove 19 is formed by an alcove partition 27 which occludes a major portion of the area of the waveguide 20. The alcove 19, in the preferred embodiment, is shown as a wedge-shaped alcove having an entrance aperture, and which decreases in area in the direction of the short circuited waveguide end 18. An apertured surface 15a, as shown in Figures 6A, 6B, is provided on the end of center conductor 15, creating a stream of air for cooling the electrodeless lamp 11. The apertured surface 15a is curved and has a center of curvature common to the electrodeless lamp 11 center of curvature. This provides a constant distance between the end of the enter conductor and the surface of electrodeless lamp 11. Since there is little RF electric field in the alcove 19, the RF magnetic field filling the space is constant, and equal to the value of the field tangent to the end of the waveguide 20. The coupling loop, excited by this field, is bounded by the middle of the center conductor 15, the upper waveguide wall and the alcove partition 27, and has a typical area of 50 square millimeters. Such a small loop couples effectively only to low impedances. The coupling from the waveguide to the coaxial transmission line would provide a voltage reflection coefficient in the waveguide greater than .8 if the coaxial transmission line was terminated in its own characteristic impedance, instead of the electrodeless lamp. A conventional coupling loop for joining the waveguide to the characteristic impedance of a coaxial line, typically 50 Ohms., would have about 10 times more area. Figures 4 and 5 illustrate yet other embodiments of the coupling device in accordance with the present invention. Figure 5 is a top view of Figure 4, and illustrates that the alcove 19 can be of rectangular shape, and defined by a partition 27. In all cases, sufficient clearance must be left between the top sidewall and the center conductor 15 to avoid arcing. Figures 6A and 6B illustrate the tip of the center conductor 15 having the plurality of holes 15a. As can be seen, there is a radius of curvature on the top surface to maintain a constant distance between the center conductor and the surface of the electrodeless lamp. Figures 7A, 7B and 8 illustrate other embodiments of the invention, all of which use a rectangular alcove structure 19 for providing a transition between transmission line 12 and the microwave waveguide 20. The alcove structure 19 of figures 7A and 7B has a reduced width, as opposed to the full width of the rectangular waveguide. Figure 8 shows an alcove structure 19 which extends perpendicular to the main axis of the waveguide 20. The operation of the coupling mechanism can be explained with reference to Figures 9, 10 and 11. Figure 9 illustrates an equivalent discrete circuit showing how the electrodeless lamp 11, which is essentially a resistive load, is capacitively coupled to the transmission line 12. The resulting termination complex impedance is approximately 20-J300. Figure 10 illustrates that by adjusting the length L of the transmission line 12, the effective impedance presented to the transformer representing the coupling presented by alcove 19 is changed. Figure 11 illustrates how, by adjusting the length of the coaxial line 12, the load can be made purely resistive. Using conventional Smith chart representations, the length is increased until the load seen at the opposite end of the transmission line is purely resistive. Using the electrodeless lamps of the current assignee of the present application, the resistive component of the impedance is determined using this method as approximately 5,000 Ohms. This represents a mismatch to the coaxial line, producing a voltage reflection coefficient of 98%. This impedance reflected back to the waveguide is equivalent to a pure 5,000 Ohm resistance at a distance of 1/4 wave from the waveguide, and is seen at the waveguide as approximately one-half an Ohm. To match the coaxial line input impedance to the waveguide, the coupling is adjusted with the alcove structure to match the output impedance of the waveguide to the 1/2 Ohm coaxial line input impedance. Low coupling produces a lower output impedance, which approaches the low input impedance of the lamp terminated coaxial lines. The reflection coefficients of the two structures are complimentary and an impedance match is obtained over a limited bandwidth. In the preferred embodiment of the invention, the alcove partition occludes approximately 80% of the waveguide 20, and the width of the alcove is the same as the interior width of the waveguide 20. This can be narrowed in accordance with the embodiment represented in Figure 7B when other alcove shapes are employed. Thus, there is described a technique for coupling microwave energy to an electrodeless lamp 11 with a minimum coefficient reflection and therefore a maximum power transfer. Those skilled in the art will recognize yet other embodiments defined by the claims which follow.

Claims: What is claimed is: 1. An apparatus for coupling electromagnetic energy to an electrodeless lamp comprising: a waveguide having one end connected to a source of electromagnetic radiation, and a closed second end; a coupling device at said second end for coupling microwave energy from said waveguide including: an alcove partition adjacent said closed end which occludes a major portion of said waveguide in the region of said second end forming an alcove with one wall of said wall of said waveguide; and, a coaxial transmission line having a center conductor extending through said one wall of said waveguide contacting a surface of said alcove partition, coupling microwave energy from said waveguide to an exterior end of said center conductor. 2. The apparatus for coupling electromagnetic energy of claim 1, wherein said coaxial transmission line center conductor extends through said alcove partition and exits a second wall of said waveguide. 3. The apparatus of claim 2, wherein said coaxial transmission line center conductor is connected to a source of gas for cooling a lamp spaced opposite an opposite end of said center conductor. 4. The apparatus of claim 3, wherein said center conductor end opposite said lamp has an apertured surface which provides gas to said lamp. 5. The apparatus of claim 4, wherein said apertured surface is curved along a radius which extends through the center of said lamp. 6. The apparatus of claim 1, wherein said coaxial transmission line has an outer conductor which terminates on said one wall and is substantially transparent to light produced from said electrodeless lamp. 7. The apparatus of claim 1 wherein said alcove partition includes an inclined surface opposite said one wall which defines said alcove in the form of a wedge. 8. The apparatus of claim 1 wherein said alcove partition is a surface opposite said one wall extending to said closed end defining a rectangular alcove. 9. An apparatus for coupling electromagnetic energy to an electrodeless lamp comprising: a waveguide having one end connected to a source of electromagnetic energy; and, a coupling member connected to a remaining end of said waveguide defining an alcove having a smaller cross- sectional area than said waveguide cross-sectional area; and, which includes a coaxial center conductor contacting a wall of said alcove and extending through an opening in a wall of said waveguide to a free end adjacent said electrodeless lamp. 10. The apparatus of claim 10, wherein said coupling member includes a coaxial outer conductor which encloses said center conductor which terminates at said wall through which said center conductor extends. 11. The apparatus of claim 11 wherein said coaxial outer conductor is transparent to light. 12. The apparatus of claim 10, wherein said center conductor passes through said contacting wall of said alcove and is connected to a source of cooling gas. 13. The apparatus of claim 13, wherein said center conductor free end includes an apertured end which directs said cooling gas to a surface of said electrodeless lamp. 14. The apparatus of claim 14, wherein said apertured surface is curved along a radius. 15. The apparatus for coupling electromagnetic energy of claim 9, wherein said coupling member provides coupling between said waveguide and said coaxial transmission line, which produces a voltage reflection coefficient in said waveguide greater than .8 when said coaxial transmission line is terminated in its own characteristic impedance.

Inventors: SIMPSON JAMES E, SIMPSON, JAMES, E.
Applicants: FUSION LIGHTING INC, FUSION LIGHTING, INC.
Application year: 1994
Main classification: H01P5
IPC codes: H01J65, H01P5, H05B41